Statistical mechanics of noncovalent bonds in polyamino acids. IX. The two‐state theory of protein denaturation

The two‐state theory of protein denaturation, in which it is assumed that a protein exists as two species, native and denatured, is examined by three approaches in this paper. First, the point is made that denaturation of an isolated molecule involves a continuous shift in the probabilities of occurrence of many states generated by the partition function for the protein. It is argued that the maximum term approximation does not lead to a two‐state mechanism of denaturation, and that the extent of cooperation implied by the two‐state theory should give very much sharper transitions than are actually found in proteins. Second, the two‐state theory is applied to the various model systems treated in this series of papers, and is found to be inadequate. Since the detailed behaviour of the models is known, it is possible to deduce the effect of the incorrect application of the two‐state theory to a system that “denatures” by gradual unwinding. This exercise is useful when, thirdly, we examine experimental data that seem to depart radically from a two‐state interpretation. We conclude that a mechanism of gradual unwinding is the most generally valid assumption, and that the two‐state theory has no theoretical or experimental support.